Lubricant Composition for a 4-Stroke Marine Engine

ABSTRACT

The invention relates to a grade SAE 30 or 40 lubricant oil for marine four-stroke engines, comprising: (a) 50 to 95% by weight of an oil of mineral or synthetic origin, (b) at least one nanoparticulate detergent of a quantity sufficient for the neutralisation of the acid compounds formed by oxidation of the sulphurous compounds in the fuel supplying the engine, (c) 1 to 20% by weight of at least one polyisobutylene which is soluble in the base oil, with a number average molecular weight of 500 to 8000 inclusive and a kinematic viscosity (at 100° C.) of 50 to 50,000 mm 2 /s inclusive, preferably 200 to 6,000 mm 2 /s, (d) 0.05 to 5.0% of at least one aminated anti-oxidant agent active at a temperature of or above 180° C. and the use of said oil for lubrication of a four-stroke marine engine.

The present invention relates to a novel lubricating oil for 4-stroke marine engines, especially for semi-fast 4-stroke marine engines, which contains the combination of at least one polyisobutylene and at least one heat-resistant amine antioxidant.

Two types of 4-stroke marine engines are generally distinguished, namely fast and semi-fast 4-stroke marine engines. The engines of the first type are engines with a low to moderate power range (15 to 200 kW per cylinder) derived from terrestrial engines and using fuels of the distillate type, such as marine diesel with a low sulfur content. Their operating speed is in general around 1200 rpm. These engines are used for propelling low-tonnage ships and as electricity generators on board larger ships.

Semi-fast 4-stroke marine engines are engines with a moderate to high power range (500 to 2000 kW per cylinder) of design similar to that of fast 4-stroke engines, but which differ from the latter by the large size of the piston/cylinder assembly. These engines are furthermore distinguished by the fact that the crown of the piston is cooled by the circulation of a coolant and by the fact that they can rotate equally well in either direction. These engines use in general a residual fuel called bunker fuel oil or heavy fuel oil which, owing to its high sulfur content, requires a lubricant having a high total base number, generally between 30 and 65 mg of KOH/g of lubricant. The operating speed of semi-fast 4-stroke marine engines is between 300 and 600 rpm. These engines are used for propelling many ships, such as cargo ships, oil tankers, ferries and even certain container ships. They may also be used as electricity generators on board large ships or in diesel-fired electric power stations.

These 4-stroke marine engines have a very different operation to that of 2-stroke marine engines, in particular as regards their method of lubrication. This is because 2-stroke marine engines are very slow engines, with a high to very high power range (2000 to 6000 kW per cylinder). These engines always consist of two separately lubricated parts, namely the piston/cylinder assembly lubricated with total-loss lubrication by the highly viscous cylinder oil, generally of SAE 50 or 60 grade, and the crankshaft lubricated by a less viscous oil system, generally of SAE 30 grade.

In general, these 2-stroke engines use a residual fuel called heavy fuel oil which, owing to its very high sulfur content, generally requires cylinder oils with a total base number (BN) possibly up to 100 mg of KOH/g of oil.

Two parameters determine the oil change interval for completely or partly changing oils for 4-stroke marine engines, namely the change in viscosity as a function of time and the base number of the oil. This is because:

-   -   there is a systematic decrease in the base number (BN) as a         function of time, which decrease is due to the neutralization of         the acids formed during combustion of the sulfur-containing         species present in the fuel. This decrease in BN requires the         oils to be monitored since, below a certain limit, the         neutralizability of the oil is no longer sufficient to combat         the corrosive effects of the acids formed, which may result in         excessive corrosive wear, in particular of the piston/liner         assembly; and     -   there is a systematic increase in the viscosity of the oil as a         function of time, which increase is due to oxidation of the         constituents of the oil, but the precise mechanisms of which are         still poorly understood. This increase in the viscosity requires         viscometric monitoring of the oils. This is because, above a         certain viscosity, the oil can no longer satisfactorily fulfill         its role of lubricating the moving parts or its role of         heat-transfer fluid for controlling the temperature of the         piston crown.

A warning procedure, consisting in regularly taking engine oil samples and subjecting them to viscometric analysis and BN measurement is thus carried out on most ships using 4-stroke engines. For this warning procedure, it is generally estimated that a partial or complete oil change is necessary when the kinematic viscosity of the oil measured at 40° C. exceeds a value of 200 mm²/s for an SAE 40 grade oil or when the BN of the oil goes below half the initial value.

Of these two parameters, the increase in viscosity is actually the critical parameter governing the oil change interval for 4-stroke marine engines.

The objective of the present invention is to slow down as much as possible the increase in viscosity of a lubricating oil used in a fast or semi-fast 4-stroke marine engine so as to extend the maximum interval between two oil changes, which presently is of the order of 10 000 hours for the known oils.

Within the context of its research on slowing down the increase in viscosity of oils for 4-stroke marine engines and of thus extending the oil change interval for these oils, the Applicant has discovered that the association of a particular synthetic polymer thickener and of at least one amine antioxidant active at high temperature makes it possible to considerably slow down the increase in viscosity of lubricating oils for 4-stroke engines.

Consequently, one subject of the present invention is an SAE 30 or 40 grade lubricating oil for 4-stroke marine engines, comprising:

-   -   (a) 50 to 95% by weight of a base oil of mineral or synthetic         origin;     -   (b) at least one nanoparticulate overbased detergent in an         amount sufficient to neutralize the acid compounds formed by         oxidation of the sulfur compounds in the fuel supplied to the         engine;     -   (c) 1 to 20% by weight, preferably between 1 and 15% by weight,         of at least one polyisobutylene soluble in the base oil, having         a number-average molecular weight between 500 and 8000,         preferably between 900 and 3000, and a kinematic viscosity (at         100° C.) between 50 and 50 000 mm²/s, preferably between 200 and         6000 mm²/s; and     -   (d) 0.05 to 5.0% of at least one amine antioxidant active at a         temperature of 180° C. or above.

Another subject of the invention is the use of such an oil for lubricating a 4-stroke marine engine.

A final subject of the invention is the use of the combination of at least one polyisobutylene soluble in the base oil, having a number-average molecular weight between 500 and 8000, preferably between 900 and 3000, and a kinematic viscosity (at 100° C.) between 50 and 50 000 mm²/s, preferably between 200 and 6000 mm²/s, and at least one amine antioxidant active at a temperature of 180° C. or above, in order to slow down the increase, over the course of time, of the kinematic viscosity of a 4-stroke marine engine lubricating oil comprising a base oil of synthetic or mineral origin and at least one nanoparticulate overbased detergent.

The system of classifying engine oils defined by the Society of Automotive Engineers (SAE) in the United States classifies engine oils both by their cold rheological behavior (with the SAE prefix) and their viscosity at the operating temperature of an engine (SAE suffix). The table below indicates the kinematic and dynamic viscosity ranges of engine oils corresponding to the various SAE suffixes.

Kinematic viscosity Dynamic viscosity (mm²/s) measured at (mPa.s) measured at SAE 100° C. under low-shear 150° C. under high-shear suffix conditions conditions 20  5.6 − 9.3 >2.6 30  9.3 − 12.5 >2.9 40 12.5 − 16.9 >2.9* 40 12.5 − 16.9 >3.7** 50 16.9 − 21.9 >3.7 60 21.9 − 26.1 >3.7 *for 0W40, 5W40 and 10W40 oils; **for 15W40, 20W40 and 25W40 oils.

This table shows that each SAE suffix corresponds to a well-defined kinematic viscosity range that does not overlap the kinematic viscosity range of the higher or lower SAE suffix.

The lubricating oils for 4-stroke marine engines of the present invention are SAE 30 or 40 grade oils, that is to say those having a kinematic viscosity, measured at 100° C., between 9.3 and 16.9 mm²/s and a dynamic viscosity measured at 150° C. of greater than 2.9 mPa·s.

In a preferred embodiment, the engine oils according to the invention have a kinematic viscosity (100° C.) between 10 and 15 mm²/s and more particularly between 13.5 and 14.5 mm²/s.

The base oil serving for preparing the lubricating oils of the present invention may in principle be any base oil, of mineral or synthetic origin, commonly used in the composition of lubricating oils.

These base oils are presently classified in five groups, numbered I to V, defined by the API (American Petroleum Institute) as a function of the characteristics indicated in the following table:

Content of saturated Sulfur Viscosity index compounds content (VI) Group I  <90%  >0.03% 80 ≦ VI ≦ 120 Group II ≧90% ≦0.03% 80 ≦ VI ≦ 120 Group III ≧90% ≦0.03% ≧120 Group IV poly (α-olefins) Group V Other bases (for example esters and alkylbenzenes)

The viscosity index (VI) is determined according to the ASTM D 2270 standard from the kinematic viscosities measured at 40° C. and at 100° C.

For economic profitability reasons, the base oil is preferably an oil of mineral origin, that is to say an oil obtained by refining crude oil, belonging to one of the above groups I to III.

Base oils of groups II and III have, owing to their high content of saturated compounds, a relatively lower polarity than the base oils of group I, thereby resulting in a lower detergent power. Moreover, the additive contents in marine engine oils are generally quite high and might not be sufficiently soluble in low-polarity bases of groups II and III. For the above reasons, base oils of mineral origin of group I are particularly preferred for preparing the lubricating oils of the present invention.

Among base oils of group I, a distinction may be made between distilled bases, obtained by specific refining of distillate cuts coming from the vacuum distillation unit, and brightstock base oil, residual base oil grade obtained by specific refining of the short residue of the vacuum distillation unit. Brightstock base oil has a kinematic viscosity of 100° C. of greater than 25 mm²/s, usually greater than 30 mm²/s, whereas distilled base oils, called neutral solvent oils, are classified by their SUS viscosity at 100° F., which varies from 100 to 800 SUS units.

The mixture of base oils used in the invention preferably contains little or no brightstock. This is because the use of polyisobutylene as thickening agent makes it possible to partly or even completely replace this ingredient conventionally used for increasing the viscosity of mineral lubricating oils. Total or partial replacement of brightstock with polyisobutylene advantageously results in less engine fouling, since the degradation of polyisobutylene leads to the formation of volatile products that escape from the engine and not, as in the case of brightstock, to the formation of dark colored carbonization residues liable to form deposits or coatings on the various parts of the engine.

The mixture of base oils represents from 50 to 95% by weight, preferably 70 to 90% by weight, of the engine oil according to the invention.

The viscosity index of the base oil, determined according to the ASTM D2270 standard, is preferably greater than 80, in particular between 95 and 110.

The engine oils according to the invention furthermore contain at least one nanoparticulate overbased detergent. Overbased detergents are organomineral nano-particles consisting of a mineral core made of calcium, magnesium, barium or sodium carbonate adsorbed on the surface of which is a layer of associated surfactants with a reverse micelle structure (hydrophobic tail directed toward the outside of the micelle and hydrophilic head directed toward the inside of the micelle). Overbased detergents are preferably calcium carbonate phenates, salicylates and/or sulfonates. They are known and used conventionally as neutralization and/or detergent agents in engine oils. Overbased detergents ensure that the engine is clean at high temperature and limit corrosive wear of the engine by the acids formed by oxidation of the sulfur compounds in the fuel.

The amount of overbased detergent in the oils of the present invention is expressed in the form of a base number (base number (BN) or total base number (TBN), determined according to the ASTM D-2896 standard). This is generally between 3 and 100 mg of KOH/g.

The amount of overbased detergent used in the oils of the present invention depends of course on the sulfur content of the fuel used. The higher the sulfur content, the greater must be the amount of overbased detergent.

Thus, for fuel such as heavy fuel oil having a high sulfur content, generally between 0.2 and 4.5% by weight, it is necessary to add a sufficient amount of overbased detergent to the engine oil according to the invention in order to obtain a base number between 20 and 65 mg of KOH/g.

However, for engines supplied with fuels having a low sulfur content (0.05 to 0.2% by weight), such as fast 4-stroke marine engines, a base number between 3 and 20 mg of KOH/g is generally sufficient.

These ranges are however given merely by way of indication and can be easily modified by the oil formulator according to the sulfur content of the fuel used in the engine in question.

The polyisobutylene (PIB) used in the lubricating oils of the present invention is a viscous liquid miscible with the base oil. As indicated above, it has a number-average molecular weight between 500 and 8000, preferably between 900 and 3000, and a kinematic viscosity (at 100° C.) between 50 and 50 000 mm²/s, preferably between 200 and 6000 mm²/s.

Polyisobutylene is the main thickening agent of the oils of the present invention and will consequently be used to adjust their viscosity. A person skilled in the art would have no difficulty in choosing, from among the ranges indicated above, the amount of polyisobutylene according to its molecular weight or, conversely, the molecular weight of the polyisobutylene according to the amount of PIB that he desires to incorporate into the oil—for a given final viscosity, the amount of polyisobutylene used must be greater the lower the molecular weight of the polymer, and vice versa.

Polyisobutylene is a product commercially available from many manufacturers.

The term “polyisobutylene” as used in the present invention also covers mixtures of several polyisobutylenes, synthesized separately and possibly having molecular weights outside the ranges of values indicated above, provided that the mixture of the various PIBs has a molecular weight lying within said ranges.

The antioxidants used in the present invention in association with the polyisobutylene, in order to slow down the increase in viscosity of the lubricating oils over the course of their use, must withstand the high temperatures routinely encountered in 4-stroke marine engines, that is to say they must always be active at temperatures above 180° C. Furthermore, the antioxidants must be soluble in the lubricating oil of the invention.

The Applicant has also found that amine antioxidants, preferably aromatic amines, are the only families of antioxidants to give, in combination with a polyisobutylene thickening agent, useful results in terms of slowing down the increase in viscosity of lubricating oils. As examples of aromatic amines, mention may be made of aromatic monoamines of formula:

R¹R²R³N

where R¹ and R² each represent, independently of each other, a hydrogen atom, a C₁₋₂₀, preferably C₄₋₁₆, aliphatic group, or an aromatic or heteroaromatic, monocyclic or condensed polycyclic, substituted or unsubstituted group, R³ is an aromatic or heteroaromatic, monocyclic or condensed polycyclic, unsubstituted group or a group carrying a C₁₋₂₀ alkyl substituent, or R¹ and R³ together form an aromatic or heteroaromatic, monocyclic or condensed polycyclic group.

The amine antioxidants according to the invention include for example the diphenylamine, phenylnaphthylamine, phenothiazine, imidodibenzyl and N,N′-diphenyl(phenylenediamine) families.

Preferred aromatic amines that may be mentioned as examples are those corresponding to the following formulae:

Among these aromatic amines, alkylarylamines are preferred.

The amount of amine antioxidant is between 0.05 and 5.0% by weight, preferably between 0.1 and 1.0% by weight.

The lubricating oil according to the invention furthermore generally contains an additive concentrate comprising for example, among other things, dispersing agents, antiwear additives, anticorrosion agents, anti-rust agents, antifoam agents, antioxidants other than the amines described above, and agents for lowering the flow point of the oil. All these additives are known and routinely used to improve the characteristics of lubricating oils.

The present invention will be illustrated by the following example, which shows the synergistic effect of the combined use of a polyisobutylene and an oxidizing agent stable at a temperature above 180° C. on the oxidated increase in viscosity of a lubricating oil in a 4-stroke marine engine.

EXAMPLE Trial of Long-Term Oxidative Aging of a Lubricating Oil as a Thin Film (See the Appended Diagram)

The operating method simulated the oxidative aging of lubricants for 4-stroke engines. A volume of about 400 ml of lubricating oil was poured into a glass vessel having an inside diameter of 115 mm. This vessel was furthermore provided with a double-walled jacket allowing the circulation of water as coolant. Throughout the length of the trial, the temperature and the flow rate of the cooling water were controlled so that the temperature of the lubricant inside the vessel did not exceed about 60° C. The entire device was inclined at 3° to the horizontal.

An aluminum test piece was placed in this vessel so that the circular bottom of the test piece (with a diameter 105 mm) was 80 mm from the internal bottom of the vessel and about 40 mm from the level of the test oil. This test piece was kept at the test temperature, that is to say a temperature between 270 and 320° C., by means of an electrical resistor with a power of 200 W, inserted inside the test piece.

An opening allowing the passage of a motor-operated rotary arm, inclined at 14° to the horizontal, was made through the jacket in the side wall of the vessel. The arm extended in the space (with a height of about 40 mm) between the surface of the oil and the lower surface of the bottom of the test piece. This arm,

TABLE 1 Trial 4 Trial 5 Trial 6 Trial 7 according according according according Trial Trial Trial to the to the to the to the comp. 1 comp. 2 comp. 3 invention invention invention invention Mineral base oil 84.9% 84.6% — — — — — (600 NS) Mineral base oil — — 79.4% 79.1% 71.6% 79.1% 79.1% (330 NS) Additives* 15.1% 15.1% 15.1% 15.1% 15.1% 15.1% 15.1% PIB A^((a)) — — — — 13.0 — — PIB B^((b)) — —  5.5%  5.5% —  5.5%  5.5% Amine antioxidant (alkylarylamine) OLOA 4860 —  0.3% —  0.3%  0.3% IRGANOX L57  0.3% LUBRIZOL 5150C  0.3% Time to reach a 110 h 110 h 100 h 170 h 200 h 150 h 180 h kinematic viscosity at 40° C. of 200 mm²/s ^((a))polyisobutylene of 910 number-average molecular weight; ^((b))polyisobutylene of 2500 number-average molecular weight.

The comparison between comparative trials 1 and 2 clearly shows that the addition of 0.3% of amine antioxidant to the lubricating oil has no effect on the rate of increase of the kinematic viscosity of the oil. Similarly, nor is the rate of increase of the kinematic viscosity of the oil slowed down by merely replacing some of the base oil with polyisobutylene (comparison between comparative trials 1 and 3). Only by combining a polyisobutylene with an amine antioxidant is the time elapsing before the kinematic viscosity reaches the critical value of 200 mm²/s considerably extended (comparison between trials 4 and 5 according to the invention and comparative trial 3).

Trials 4, 5, 6 and 7 according to the invention demonstrate that several commercial antioxidant additives of alkylarylamine structure allow the time that elapses before the kinematic viscosity reaches the critical value of 220 mm²/s to be considerably and equivalently extended. 

1. An SAE 30 or 40 grade lubricating oil for 4-stroke marine engines, comprising: (a) 50 to 95% by weight of a base oil of mineral or synthetic origin; (b) at least one nanoparticulate overbased detergent in an amount sufficient to neutralize the acid compounds formed by oxidation of the sulfur compounds in the fuel supplied to the engine; (c) 1 to 20% by weight of at least one polyisobutylene soluble in the base oil, having a number-average molecular weight between 500 and 8000 and a kinematic viscosity (at 100° C.) between 50 and 50 000 mm²/s, preferably between 200 and 6000 mm²/s; and (d) 0.05 to 5.0% of at least one amine antioxidant active at a temperature of 180° C. or above.
 2. The lubricating oil as claimed in claim 1, characterized in that the amine antioxidant active at a temperature of 180° C. or above is an aromatic amine.
 3. The lubricating oil as claimed in claim 1, characterized in that it has a kinematic viscosity at 100° C. of between 10 and 15 mm²/s, preferably between 13.5 and 14.5 mm²/s.
 4. The lubricating oil as claimed in claim 1, characterized in that it has a total base number between 3 and 100 mg of KOH/g.
 5. The lubricating oil as claimed in claim 1, characterized in that the overbased detergent or detergents are chosen from phenates, salicylates and sulfonates.
 6. The lubricating oil as claimed in claim 2, characterized in that the amine antioxidant agent is an aromatic monoamine of formula: R¹R²R³N where R¹ and R² each represent, independently of each other, a hydrogen atom, a C₁₋₂₀, preferably C₄₋₁₆, aliphatic group, or an aromatic or heteroaromatic, monocyclic or condensed polycyclic, substituted or unsubstituted group, R³ is an aromatic or heteroaromatic, monocyclic or condensed polycyclic, unsubstituted group or a group carrying a C₁₋₂₀ alkyl substituent, or R¹ and R³ together form an aromatic or heteroaromatic, monocyclic or condensed polycyclic group.
 7. The lubricating oil as claimed in claim 6, characterized in that the amine antioxidant is an alkylarylamine.
 8. The use of an oil as claimed in claim 1 as a lubricating oil for 4-stroke marine engines.
 9. The use of the combination of: (a) at least one polyisobutylene soluble in the base oil, having a number-average molecular weight between 500 and 8000 and a kinematic viscosity (at 100° C.) between 50 and 50 000 mm²/s; and (b) at least one amine antioxidant active at a temperature of 180° C. or above, in order to slow down the increase, over the course of time, of the kinematic viscosity of a 4-stroke marine engine lubricating oil comprising a base oil of synthetic or mineral origin and at least one nanoparticulate overbased detergent. 